The other side of clock-watching

There was a time and a lovely time it was when the rules were very clear: students sitting in class awaiting the arrival of a professor waited 15 minutes for a Ph.D. and 10 for someone with a Master's degree. Other types of credentials did not enter into the scheme.

Now I'm on the other end of the equation and I don't know what the rules are. How long am I supposed to wait for a student to show up for an important appointment? Should I distinguish between reliable students and the habitually tardy? Should I tolerate a longer wait on a dull day and a shorter waiting on a busy day? Does it matter how badly I'm itching to get out of the office? What about the weather?

If I establish a policy on waiting, will I just be enabling the irresponsible to remain that way? But if I don't establish a policy on waiting, how do I know when to quit?

I'm waiting for answers...but I don't know how long.